Generally, time changing devices of conventional timepieces are disposed on one surface of a base plate which confronts the dial of the watch or on the opposite surface of the base plate confronting the back cover of the watch. The first mentioned type of arrangement although more popular cannot avoid an increase in the spacing between the externally operable hand adjustment shaft and the dial because the time changing device overlaps a calender member and the like operated by a clutch wheel engaged with a calendar member and the adjustment shaft which should necessarily be positioned to confront the dial. Accordingly, although the watch mechanism can be housed in a thin casing the finished timepiece has its winding crown positioned not in the expected central area of the casing but closer to the back cover of the casing owing to the excessive length of the adjustment shaft. This is detrimental to the appearance or design which is one of vital conditions of a thin timepiece.
FIG. 1 illustrates the second mentioned type of arrangement in which the time changing device is mounted on the opposite side of the base plate i.e. the surface confronting the back cover.
FIG. 1 is a plan view of the mechanism of a conventional electronic timepiece. The time changing device has its position in a horizontal plane restricted by various components of the watch including a battery 1, a driving coil 2, a circuit board 3 carrying IC, quartz vibrator and like circuit components, and a gear train bridge 4. A setting lever 6 is operatively connected with the adjustment shaft 5 and lever 6 has its angular position controlled by a cam portion 7b provided at the end of a resilient extension arm 7a of a setting lever spring 7. The resilient extension arm 7a allows the cam portion 7b to flex which occurs when a cam follower comprising a pin 6a upstanding from the setting lever 6 passes over the cam surface of portion 7b. Another function of the extension arm 7a is to provide a `snap` action for the shaft 5 into and out of the casing of the watch. Preferably, the resilient extension arm 7a is formed as long as space permits from the aspect of strength and also as wide as possible from the viewpoint of machinability. A fixed body part 7c of the spring 7 is positioned above the battery 1 as viewed in the drawing to ensure a sufficient length of the resilient extension arm 7a.
Thus, the arrangement mentioned above is inferior to the first mentioned arrangement as regards horizontal space since the battery 1, quartz vibrator, circuit board 3, gear train, transducer and like components are arranged on that surface of the base plate 10 confronting the back cover. Moreover, the adjustment shaft 5 which is manipulatable for the correction of the hands of the watch and calender has its axially outward position limited by the spring 7 with a snap action. To limit outward movement of shaft 5 the spring 7 includes the cam portion forming a leading end of the resilient arm and the pin 6a fixed on the setting lever 6 is movable into and out of the position limiting recesses of the cam portion. To provide the snap action, the pin mentioned above moves over the ridge of the cam portion thereby flexing the resilient arm. Thus, the durability and reliability of the spring 7 must be enhanced by making the resilient arm as long as possible. This brings about another cause of the difficulty which is experienced in disposing the time changing device on the side of the base plate confronted by the back cover. More specifically, the distance between the axis of locating pins 8 and 9 on the spring 7 is so short that the positional accuracy of the cam portion 7b is poor. This is because the horizontal space surrounding the battery has not been used effectively.